This invention relates to improvements in reciprocating compressors, and in particular, to a system to reduce the load on the compressor upon initial startup thereof.
Reciprocating compressors are employed in many varied conditions. In a number of applications, the compressor is designed to operate under a constant load. Accordingly, an electric motor typically employed to drive such compressor may be chosen to provide sufficient torque to handle the load imposed thereon, even during initial starting conditions. It is understood, the motor employed to drive the compressor will not provide maximum torque until it attains normal operating speed.
In other applications, the compressor operates in systems wherein a variable load may be imposed thereon. An electric motor may be chosen to provide sufficient torque at starting conditions to manage the maximum load that might be imposed on the compressor. However, the use of such a motor would not be economical. The relatively few times the motor would be employed under such adverse conditions does not warrant the increased cost in providing windings capable of carrying the large flow of current that would occur during the starting period of a compressor having a relatively large load imposed thereon.
The use of a compressor in a refrigeration unit is a typical variable load application. At times when the ambient temperature is at a relatively high level, the condensing pressure of the refrigeration unit is similarly at a high level. Compressor discharge pressure must be slightly above condenser pressure. Accordingly, the initial load on the compressor is relatively high.
Under heavy load conditions, the motor will draw excessive current in an attempt to produce the necessary torque to attain operating speed. The excessive current may damage the motor windings or more likely, trip the safety device employed to prevent the windings from being damaged by the excessive current. If the load imposed on the compressor is unusually large, the motor may stall producing locked rotor conditions which may result in the total destruction of the motor.
In U.S. Pat. No. 1,607,657, there is disclosed a thermostatically controlled needle valve to permit the cylinder to be vented to atmosphere to facilitate starting of the compressor. The use of an expansible bellows in combination with a needle valve provides a rather costly and cumbersome arrangement to obtain the desired reduced load on the compressor.
It has been determined that an effective means for reducing the initial load on the compressor can be accomplished by communicating a portion of the compressor operating at compressor discharge pressure with a portion operating at suction pressure. This permits a portion of the relatively warm high pressure fluid to mix with the relatively cold low pressure fluid. The temperature of the suction fluid is thus increased, thereby lowering its density. By decreasing the density of the suction fluid, the compressor is required to do less work in compressing the fluid to attain discharge pressure.
Additionally, the suction pressure of the fluid is increased as a result of the introduction of fluid at discharge pressure into the suction side of the compressor. By increasing the suction pressure of the gas, the pressure differential across the compressor is reduced, thereby further decreasing the starting load thereon.